<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 300px;
            height: 300px;
            background-color: #ccc;
            /*边框*/
            /*border-width: 20px;
            border-style: solid;
            border-color: red;*/
            border: 20px solid red;
            padding:30px;
            /*自动内减*/
            box-sizing: border-box;
        }
    </style>
</head>
<body>
<div>

</div>
</body>
</html>